A well known type of AC drive includes an AC-to-DC converter including a boost rectifier for converting three-phase AC source voltages to DC voltages on a DC bus. The DC bus interfaces the AC-to-DC converter to a DC-to-AC inverter, which is typically a three-phase bridge network of solid state switches, which are switched at high frequency to generate pulse width modulation (PWM) or other types of modulated low frequency power signals which are supplied to an AC motor. These systems generate a common mode voltage, for example, a voltage measured between a neutral in the motor and an electrical ground. These also generate common mode currents in part the result of parasitic capacitances between mechanical parts in the motor and ground, and between mechanical parts in the motor and the stator windings. It is desirable to attenuate or eliminate these voltages to prevent interference that might trip fault protection devices and to reduce currents in motor bearings that might reduce their service life. Passive circuits including filters and transformers have been employed to correct this problem, but with increased production costs and increased installation costs. A number of prior art publications have suggested modifications to inverter modulation methods to control the inverter common mode voltages. This approach has cost and manufacturing advantages over passive circuits.
The inverter switching states can be modeled with the aid of a space vector PWM (SVPWM) theory and diagram more fully described below. Two of the vectors in this theory are zero-voltage switching vectors (V0 and V7). Some prior art methods skip these vectors by using two active vectors that are 180 degrees out of phase. However, these modified modulation schemes require that dwell time (on time for the inverter switches) be calculated in real time.
Some modified modulators shift the active voltage vectors of the inverter to align with those of the boost rectifier. With this strategy, it is possible to eliminate one common-mode voltage pulse in every switching period by shifting the active voltage vectors of the inverter to align with those of the boost rectifier. Compared with the conventional three-phase SVPWM methods, this proposed method can reduce the total number of common-mode voltage pulses to two-thirds. However, this SVPWM strategy cannot be applied to diode front-end variable frequency drive (VFD) systems that are more common in AC motor drives. For those modified modulators for active front-end VFD systems, dwell times are calculated in real time to shift the active voltage vectors, and those shifts are performed in each switching period.
It would be advantageous to provide common-mode voltage reduction methods for a PWM carrier-based modulator by eliminating zero-voltage vectors.